1. Field of the Invention
The present invention relates to a digital oscillator and to a supply circuit for an eddy current probe.
2. State of the Art
A standard, multifrequency eddy current probe, as described in the work entitled "Non Destructive Testing Handbook" (MacMaster R. C. (ed), American society for Non Destructive Testing, 1986, vol. 4) and more particularly in sections 20 "Concepts of Multifrequency Eddy Current Testing" (R. Saglio, Intercontrole Inc., Rungis, France and M. Pigeon, Commissariat a l'Energie Atomique, France) and 11 "Electronic Analysis Circuits for Eddy Current Test" (E. J. Strauts, Magnaflux Corporation, Chicago, Ill., USA) has a mimic diagram like that shown in FIG. 1. It successively comprises a balancing oscillator 10, an injector 11, as well as a demodulator 12 supplying a digital output SN.
The function of the oscillator is to produce a multifrequency signal on the basis of a sum of 1 to N pure sinusoids. The balancing means serves to minimize the influence of carriers in order to amplify in relative manner the modulating signal (eddy current signal).
The injector 11 supplies a sensor 13 in which the modulation by eddy currents takes place. The function of the demodulator 12 is to extract the modulating signal (modulation by eddy currents) from the carriers used.
In such an apparatus, the technical data are generally as follows:
the sinusoidal carriers have a frequency between 1 KHz and 4 MHZ, PA1 a measuring apparatus currently uses between one and four carriers simultaneously, PA1 the pass band of the eddy current signals does not exceed 1 KHz, PA1 the modulation by eddy currents is a complex modulation: amplitude and phase, PA1 the demodulator 12 has the function of extracting these two informations for each of the carriers and extracts in Cartesian form the modulated signal in the sensor having the following equation: EQU Xcos(wt)+Ysin(wt). (the demodulator extracting X and Y.) PA1 the choice of frequencies is limited, so that at a given time all the frequencies which can be generated are stored in the PROM, so that if the user requires a frequency not in the memory, he must stop the apparatus, remove the PROMs from the oscillator card and replace them by another set of circuits, PA1 it is necessary to have a system like that shown in FIG. 2 for each frequency to be generated and four systems are required for a standard, multifrequency eddy current apparatus, PA1 it is not possible to generate any random frequency, the possible frequencies being given by the formula: ##EQU1##
One way of implementing the oscillator function involves oscillating an operational amplifier-based circuit. The generated frequency then depends on the values of the passive components connected around the integrated circuit. The advantage is that the frequency produced can be accurately regulated with the aid of a simple potentiometer. The disadvantages are that the frequency regulation is manual, the characteristics of the signal produced (frequency, amplitude, quality) are sensitive to measurement-external parameters, such as temperature and an oscillator card is required for each carrier to be generated.
This oscillator can be a digital oscillator, like that described in the work "Circuits integres et techniques numeriques" by R. Delsol (Editions SUP'AERO, pp 282-286, cf. particularly fig. XIII-8) having the structure shown in FIG. 2. It then comprises a quartz oscillator 20, a counter 21, a PROM 22, a digital-analog converter 23 and a filter 24.
The quartz oscillator 20 ensures a very high stability of the frequency produced. The counter 21 is a programmable counter, whose outputs constitute the address bus of the PROM. The PROM 22 is a system of nonvolatile memories erasable by ultraviolet radiation and consequently reprogrammable. These memories contain samples of a certain number of sinusoids digitized at the frequency of the quartz oscillator. The converter 23 is a fast, 12 bit digital-analog converter, being followed by a low-pass filter 21 responsible for eliminating the frequency of the quartz oscillator.
This oscillator has the advantage of not deriving as a function of the temperature or other external events. However, it suffers from the following disadvantages:
P is the number of periods of digitized sinusoids,
F.sub.q is the frequency of the quartz oscillator,
N is the number of digitized points,
P and N are integers.
Thus, it is possible to generate more low frequencies than high frequencies. The principle is to divide F.sub.q by an integer. The resolution is more precise when in the high values of N, giving e.g.: P=1: a single sinusoid period is described,
F.sub.q =10 MHZ,
N=4 giving a frequency of 2.5 MHZ, whilst for N=5 it is 2 MHZ or 500 KHz variation for a single step. Conversely, for N=400 it is 25 kHz and for N=401, 24,938 Hz, i.e. a variation of only 62 Hz for a single step. It is impossible to generate frequencies between 2 and 2.5 MHZ. This limitation can be prejudicial in the case of the use conditions of an eddy current apparatus.
Another type of digital oscillator is shown in FIG. 3. It comprises a quartz oscillator 30, a counter 31 followed by four memories 32, an adder 33, a digital-analog converter 34 and a filter 35.
On this occasion the counter 31 controls four address buses. The memories 32 are of the volatile type. They are loaded by digitized sinusoids, as for the PROMs. For each clock stroke of the quartz oscillator 30 an addition takes place between the four data buses of the memories. The addition takes place by a real time calculation unit. The output of the oscillator is the same as in the previous case.
This oscillator has the advantage of being able to generate a sum of signals and of being reprogrammable by parametrizing. A microprocessor installed in the apparatus is able to recalculate the samples to be stored in the memories. However, it has the disadvantage of a cumbersome structure with a complex control of the different buses and the different programmable counters.
Another prior art document, U.S. Pat. No. 4,283,768, describes a digital signal generator making it possible to generate complex wave shapes of multiple frequencies and selected phases in the time range.